FIELD OF THE INVENTION
The present invention relates to a papermakers dryer fabric.
Generally speaking papermaking machines are made up of three sections, namely the forming, pressing and drying sections. By the time the paper web enters the drying section from the pressing section as much as fifty percent of the water has been removed from the web. The remaining water removal then has to be completed in the dryer section. Here the paper web is carried by dryer fabrics transferring the web in succession from one to another of the rotating surfaces of batteries of steel cylinders arranged along the length of the machine and is heated by high pressure steam. It was previously considered essential for dryer fabrics to be appreciably permeable. Known dryer fabrics generally have a permeability of at least 250 cfm.
A key objective in the field of papermaking is to produce paper with uniform properties and characteristics. One of the main problems to be overcome in this respect is the fact that during the drying stage of the papermaking process the paper sheet shrinks more in the edge regions than in the center. This has an adverse effect on the physical properties of the paper as a whole, especially the dimensional stability. It is therefore desired for paper shrinkage to be as small and uniform as possible.
Techniques aimed at overcoming this problem are often termed "sheet restraint measures". One known proposal for overcoming this problem involves increasing the vacuum pressure in the dryer section suction rolls. However, this restraint is intermittent through the dryer section on successive suction rolls. Although this reduces shrinkage in the central region of the sheet, a large shrinkage difference remains between the edge sheet regions and the center of the sheet. Alternatively, it has been proposed to apply a negative pressure profile to a part of the path of the paper in the dryer section. This is unsatisfactory since sheet restraint is ideally required throughout each cycle within the dryer section.
U.S. Pat. No. 5,397,438 to Nyberg et al describes a method of ensuring sheet restraint throughout the dryer section without the need for costly vacuum or compressed air devices. This involves generating an adhesive force between the sheet and the dryer fabric. This is brought about by treating the edge regions of said fabric. In all cases the dryer fabric retains its permeability. One example of a suitable treatment method includes the incorporation of a statically chargeable material onto the fabric such that the positively charged fabric attracts the negatively charged cellulosic fibers of the sheet. Alternative treatment methods involve applying an adhesive material to the fabric or roughening or embossing the edge regions of the fabric. These treatments are somewhat complicated to achieve in a precise manner.
The present invention therefore seeks to provide an alternative papermakers fabric that provides sufficient sheet restraint properties.
According to the present invention there is provided a papermakers dryer fabric operable to carry a paper web on one face thereof. The fabric comprising a base substrate that is coated with and/or at least partially impregnated with resin so as to yield a substrate having a substantially impermeable surface on the paper carrying side of the fabric.
By "substantially impermeable" we mean a permeability of less than 50 cfm.
The base substrate is ideally partially or fully impregnated with resin.
Preferably, the paper carrying side of the fabric is essentially smooth. It has been found that the paper web adheres well to the substantially impermeable and essentially smooth surface of the fabric as the fabric passes through the dryer section.
Although superior adhesion of the paper web to the substantially impermeable coating is achieved by providing an essentially smooth surface on the paper-carrying side of the fabric, for some applications the surface of the belt may be provided with micro-profiles such as grooves, bumps, ridges, dot patterns or roughened regions, across all or part of the fabric width.
Since nearly 70% of all paper web shrinkage takes place in the last one third of the dryer section, the fabric according to the invention is ideally solely located in the latter dryer group runs in the section (i.e. the last third of the section) which preferably operate under closed draw conditions. These dryer fabrics may, however, be located throughout the dryer section.
The base substrate may be woven or nonwoven or a combination of woven and nonwoven materials. Suitable nonwoven materials include knitted fabrics, mesh structures, interlinked spiral fabrics, tessellated structures or combinations thereof. The base structure may have a single layer or multiple layers.
The resin may comprise any polymer which is stable to the combined forces of steam and high temperature (greater than 110.degree. C.), which are typically found in a paper machine dryer section. Preferred resins include any of the following either alone or in combination: polyester, polyamide, polyamide-imides, polyimides, silicones, phenolic resins, PEEK, polyphenylene sulphide and epoxy resins including UV-curable polymers.
The resin may be cast as one or more layers from one or both faces of the belt. Ideally the resin is applied as a single monocoque cast from one side of the belt. The resin may be provided as a coating and/or impregnation treatment. The resin film may alternatively be applied by way of lamination. Alternatively, the resin may be provided in the form of melt deformable yarns incorporated into or onto the base substrate, that have been subjected to a combination of heat and pressure to yield an essentially smooth polymer surface. Ultra sonic bonding methods may even be used to iron out a smooth surface. Abrasion resistant material may be included in the resin, for example, aramid fiber pulp or (micro) particles of glass, ceramic or frit.
The surface on the paper-contacting side of the fabric is ideally smooth compared with conventional woven dryer fabrics, typically having an R.sub.2 D surface roughness value less than 10.
The machine side of the fabric does not necessarily have to be resin-coated. If not then it is preferred for high wear resistant material yarns to predominate on the machine side and/or long machine direction floats to be present to reduce abrasion. The basecloth may be endless woven, seamed or joined by other methods such as ultrasonically welding or edge joining.
Reinforcement yarns may also be incorporated into the fabric belt. These reinforcement yarns are preferably aligned in the running direction of the belt. These further load-bearing yarns may lie on top of, underneath or in the middle (i.e. between the two membrane layers) of the membrane composite, or in any combination thereof. Such further yarns may be applied by means of a bobbin that travels across the belt in an axial direction. These yarns are preferably made from high molecular weight polyethylene, polyester, polyamide, high tenacity P.V.A. or Kevlar (RTM). The load bearing yarns increase the longitudinal strength vector in the running direction of the belt.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
In order that the present invention may be more readily understood specific embodiments thereof will now be described by way of example only with reference to the accompanying drawings in which: